I am working on seasonal migration. In this reference I found that data availability is the major problem in this field of research. Most of the work on seasonal migration is in the form of case studies.
Any study, regardless of the methods being used, will be limited by the field circumstances, and especially by access to data. In this case, it sounds like it would be even more difficult to conduct a survey (quantitative data) with this population, in comparison to a well-chosen set of qualitative interviews.
I think geography is just one of the factors determining which research methods you should use to collect your data. Here are a few:
Your ontological and epistemological beliefs: If you believe in a specific paradigm which explains what the nature of reality is (ontology), how to know that reality (epistemology), then this belief could lead you to specific methodology. Constructionism, positivism, pragmatism, or critical theory provide you different idea of how to collect your data.
Your methodological stance and specific approaches: which a specific procedure to acquire knowledge of that reality (methodology) you would use? Qualitative Research or Quantitative Research or Mixed Methods Research? And more specific approach like Qualitative Case Study, Grounded Theory Method, Life History, Narrative Study, etc. Each of these specific approaches to inquiry requires you to have different forms of research questions. For instance, you could ask What, How, and Why with case study research, but ask Who, What, Where, How much, and How many (see Yin 2014; Creswell 2007).
Data availability: If data are not available, it could be better to do qualitative research as it is more helpful in exploring new areas. In your case, data availability, is NOT a problem. I think it is better to describe the situation as: the major problem is the lack of quantitative data (Am I right?). With the availability of qualitative case studies (as you said), I don't know why you can't do quantitative surveys because this kind of research can rely well on previous studies (regardless of qualitative or quantitative research) to design your own study.
Resources including time, effort, money, and human resources: There are many types of qualitative research: narrative studies, phenomenological study, case study research, etc. Not all of them requires a long period like ethnography. Some surveys may take less time, but require a lot of effort for the team (researchers and enumerators) and also money.